This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Progress towards an HIV-1 vaccine has been stymied by the inability to induce a protective humoral response. Well-characterized neutralization epitopes are either poorly immunogenic or effectively masked on the majority of patient isolates. Newer evidence suggests that even highly masked primary isolates possess sensitive neutralization targets that are recognized by autologous patient sera and occasionally by heterologous sera. This suggests that mapping the epitopes involved may identify novel targets that are capable of inducing broadly neutralizing activities. In this project, we are identifying subtype B and C infected patients that possess broadly neutralizing activities for primary isolates, and will localize the target epitopes. We are interesting in antibody activities that mediate neutralization of both autologous and heterologous clade B and clade C viral envelope (Env) glycoproteins. A large number of patient samples obtained from both North-American and African cohorts are being screened for cross-neutralizing activities, and samples with interesting neutralizing properties will be selected for detailed characterization. It is anticipated that these studies will define novel targets that are exposed on typical neutralization-resistant primary isolates. To date, we have screened over 50 plasma samples from chronically subtype C infected subjects from a Zambian cohort against a panel of cross-clade reference Envs to quantify the potency and breadth of cross-neutralizing activity. Subtype C infected patients in general produce antibodies that can neutralize heterologous Envs with low potency, although the targets and properties of these antibodies are not yet known. We have identified one subject that possesses both broad and potent neutralizing activity against heterologous Envs and are in the process of characterizing the autologous viral Env and antibody neutralizing activity in this patient, as well as generating monoclonal antibodies.